The present invention relates to a Bragg Grating Filter Optical Waveguide Device.
Optical filters have numerous applications in optical communications and in particular, they can be used for providing wavelength selectivity and tuning in WDM and DWDM systems. For this purpose, filters based on Bragg grating structures offer near ideal filter response, and high channel isolation. Bragg grating structures are also used for compensating the optical dispersion experienced by short optical pulses traversing a length of an optical fiber.
In a fiber Bragg grating a spectral component of a wavelength xcex in the input signal is reflected back at position x when the wavelength xcex, the grating period xcex9(x) and the effective index n(x) satisfy a Bragg phase-matching condition: xcex=2n(x)xcex9(x). Different spectral components of different wavelengths are reflected at different locations and have different phase delays. For instance, when the fiber has a constant effective index of refraction n=n(x) and a linearly chirped grating period xcex9(x), the phase-matched wavelength changes with the position x according to xcex9(x) only. The variation of xcex9 according to the position x is called chirp of the grating. In a linearly chirped grating, the period xcex9 of the grating varies according to a xcex9(x)=ax+b law.
FIG. 1 shows an example for the use of a chirped Bragg grating for compensating the dispersion of a signal. An initial pulse 100 is transmitted over an optical fiber line 101.
A pulse carrying information is never perfectly monochromatic. Pulse broadening arises from transmission in dispersive fibers as the high frequencies of the impulsion spectrum travel faster than the lower frequencies. As a result the pulse 102 is much broader than the initial pulse 100. In the first approximation the frequency distribution is linear along the pulse.
The grating 104 introduces a delay depending on the wavelength of the incoming pulse which is fed in with help of an optical circulator 103. Higher wavelengths xcexL are reflected at the rear end, whereas shorter wavelengths xcexS are reflected at the far end. Thus, the shorter wavelengths travel farther within the grating and thereby experience an additional time delay with respect to the longer wavelength. In order to compensate for the linear frequency distribution along the pulse, the grating has to be linearly chirped.
The request for compensating dispersion broadening is to compress pulse width. For dispersion effects in fiber lines depend on a different parameters and on the time it is necessary to adapt the compression function of a Bragg fiber grating to this system parameters.
A dispersion tunable structure with no central wavelength shift has been proposed by R. M. Measures et. al. in 13th Ann. Conf. Europ. Fibre Optic Comm. Networks 1995, p. 38-41. This allows to control the intra-grating strain distribution by means of a tapered cantilever beam which allows the variation and control of the chirp of a Bragg grating to be varied in a precise manner over a wide range of dispersion without any shift in the grating""s center wavelength. The disadvantage of this solution results in undesirable stress induced birefringence causing a high polarization mode dispersion (PMD) because the exerted strain coupled to the fiber is nonsymmetrical.
Another problem of the disclosed solution is that the bending of this device is controlled by a micrometer translator which is slow to react an fast dispersion changes.
The underlying problem of the invention is therefore to provide a Bragg grating filter optical waveguide device which allows the tuning of the dispersion of a fiber Bragg grating in a due time frame.
A first preferable solution of this problem is a Bragg grating filter optical waveguide device which comprises a first area, a second area and a third intermediate area corresponding to the central wavelength, wherein adjusting means are provided for compressing the fiber grating between said first and said intermediate area and for elongating the fiber grating between said intermediate and said second area.
Adjusting means are designed in order to induce a linear strain along the grating. This strain along the grating is half a compression and half an elongation, and the central wavelength undergoes no change as no strain is applied at this position.
The term xe2x80x9cadjusting meansxe2x80x9d refers to any means capable of changing the chirp and therefore the dispersion of a Bragg grating associated with a determined central wavelength by exerting stress and/or strain to the grating.
The first preferable solution comprises an optical fiber provided with a linearly chirped Bragg grating. Then the adjusting means compresses the Bragg grating where the grating periods are the shortest and elongates the Bragg grating where the periods are the longest. The shorter the grating period, the higher the compression, and the longer the grating period, the stronger the elongation. When such a linear strain is applied along a linearly chirped Bragg grating, the final chirp of the grating is still linear. Moreover, the chirp, that is to say the difference between the maximum and the minimum period of the grating will increase. Therefore, the dispersion which is approximately inversely proportional to the chirp in the case of a linearly chirped Bragg grating will decrease. A non-chirped or non-linearly chirped Bragg grating can also be deformed in the same manner in order to alter the dispersion.
Preferably, the longer grating periods area is arranged in upstream direction of the shorter grating periods area with respect to the signal transmission pathway. This leads to a linearly chirped grating which compensates the linear frequency distribution along a given pulse without shifting the central wavelength of the pulse.
Advantageously, the fiber grating compression is linear. This linearity allows to match the linear chirp of the Bragg grating region, because the chirp of the fiber is directly proportional to the deformation of the grating.
In an advantageous embodiment, the fiber compression and fiber elongation have essentially identical values. Thereby, the total transverse strain is essentially compensated.
A preferable holder comprises a fiber guide which is shaped in a circular or an elliptic form.
A second solution comprises an optical fiber provided with a linearly chirped Bragg grating. The adjusting means generates a linear mechanical gradient when compressed or elongated. If the adjusting means is compressed, the Bragg grating is compressed the most where the grating periods are the shortest and compressed the least where the periods are the longest. The shorter the grating period, the higher the compression, and the longer the grating period, the weaker the compression. If the adjusting means is elongated, then the Bragg grating is elongated the most where the grating periods are the longest and elongated the least where the periods are the shortest. The shorter the grating period, the weaker the elongation, and the longer the grating period, the stronger the elongation. When such a linear strain is applied along a linearly chirped Bragg grating, the final chirp of the grating is still linear. Moreover, the chirp, that is to say the difference between the maximum and the minimum period of the grating will change. Therefore, the dispersion will also change. A non-chirped or non-linearly chirped Bragg grating can also be deformed in the same manner in order to change the dispersion.
A preferred solution of this linear mechanical gradient generating ajusting means is a flat conical holder as described in the following drawings. The linearly chirped, non-chirped or non-linearly chirped Bragg grating is bonded onto this mechanical structure. Applying stress or strain on this ajusting means induces a linear chirp in the fiber Bragg grating.
Further advantages of the present invention are explained in the following drawings.
It is understood that the aforementioned advantages and the features of the invention explained in the following, are not only used in the specifically described combination, but can also be used by a person skilled in the art in other combinations or alone, without exceeding the scope of the invention.